The invention relates to position and/or velocity transducers for providing signals in regard of the position or velocity of a movable part, in particular of a rotor of an electric motor or generator.
1. Background
There is an increasing need for stiff and compact servo motor systems. To be stiff, a servo motor system must be able to give a fast response to large and unexpected external torques. A fast response requires a very high resolution and a very low delay in the speed signal.
Analog electrodynamic tachometer generators offer a solution providing high resolution but it is difficult to build them so as to give a low delay. The generator adds a not negligible inertia to the rotor. The torsional stiffness in the mechanic coupling of the tachogenerator to the motor rotor is limited, and this gives a resonance frequency that limits the possible bandwidth of the servo motor control loop. Torsionally very stiff designs require mechanically very compact assemblies of the motor and the tacho generator. However, high resolution tacho generators should not be attached close to the motor as the stray flux from the motor windings might affect the output voltage of the tacho generator in the case where the two units are located too close to each other. Tachos also adds considerable cost and increase the size of the motor system.
Transducers that detect or sense rotary or linear motion by using at least two periodic primary signals having a phase difference in the order of magnitude of 90xc2x0 such as a resolver or an optical incremental encoder have been used for many decades of years. Resolvers and synchros can be made with very high resolutions but the costs of high resolution units are very high. The time delay from a motor speed change to the corresponding change of the detector output signal is not negligible since the torsional stiffness of mechanical coupling of the resolver to the motor rotor is limited, and this gives a resonance frequency that limits the possible bandwidth of the motor control loop. Torsionally very stiff designs require mechanically very compact assemblies of the motor and the resolver. High resolution resolvers should however not be mounted close to the motor as the stray flux from the motor windings might affect the output voltage of the resolver in the case where the two units are located too close to each other.
Incremental transducers are commonly used to give angular position information. By reading position data repetitively at known time intervals, an approximate value of the velocity can be obtained. The primary signal from optical incremental transducers are normally processed in one of two ways. In the first way the analog primary signals are compared to a reference level thus converting the basic sinus signal to a square wave binary signal that is fed to a counter chain that is readable from a computer.
In the second way, the signal is fed to a xe2x80x9cmultiplyingxe2x80x9d network that digitalizes the data based on the assumption that the analog signals have a constant amplitude and are sinusoidal. Such converters can for example convert one period of a sinusoidal input signal to 5 or 100 periods of the square wave output signals.
At low speeds, the limited resolution of a digital incremental encoder gives very high quantification errors in the speed estimate. With standard decoding electronics that gives 4 count pulses per period, even a 5000 line encoder gives only 20000 positions per turn. At a relatively high speed like 15 rpm and a test interval of 200 xcexcs, the change in position is approximately 15/60*20000*0.0002=1 unit, which due to quantification errors can give either 0, 1 or 2 units as a speed estimate input signal to a control algorithm.
The resolution problem can be reduced by using encoders comprising more lines or by using interpolation circuits that generate for example 100 counts for every period of the basic encoder signal. Hardware interpolation requires a very high signal quality; the amplitude of the primary signals must be constant over one full turn and over time, and the shape of the primary signals must fit the assumptions for which the multiplier circuit has been designed, and they must do so over the full turn and over time. The linearity of these converters depends on the linearity of the primary sinus signals, and much work has been invested in different ways to obtain very linear output signals.
Both high line count encoders and encoders having a signal quality suitable for high factor interpolation put stringent demands on the light source and the mechanical properties of the encoder system. Even a 5000 line encoder normally operates with a 20 to 30 xcexcm gap between encoder disc and receiver mask pattern. Incremental encoders suitable to give a high speed resolution therefore require encoder discs having separate bearings. Such arrangements add length and cost to the motor system.
2. Prior Art
Position and velocity transducers suitable for rotating parts are for instance disclosed in the patent documents DE-A1 27 11 593, DE-A1 35 27 128, DE-A1 38 13 754, DE-A1 39 14 557, DE-C2 39 01 546, U.S. Pat. Nos. 4,990,767, 4,794,251, 4,580,047, 4,580,046, JP-A 57-169611, JP-A 63-6418.
The German patent document DE-C2 32 39 108 for Dr Johannes Heidenhain GmbH discloses a device having more than two signals of different phases, for example signals having phase differences of 0xc2x0, 30xc2x0, 60xc2x0, 90xc2x0, 120xc2x0 and 150xc2x0. Improved two phase signals can be produced by multiplying these six primary signals by factors obtained from a fourier analysis of the shape of the six primary signals and by adding/subtracting these multiplied values to obtain two new signals having a phase difference of 90xc2x0 to each other.
The European patent application EP-A1 0 541 827, also for Dr Johannes Heidenhain GmbH, discloses several arrangements for obtaining a more sinusoidal shape of the signals generated by an incremental encoder by having a pitch of the encoder mask transparent and opaque lines that is different from the pitch of the transparent and opaque lines of the encoder disc.
The company Max Stegmann GmbH has in 1994 introduced a system named SINCOS. The linearity of the sinus signals is claimed to be improved by using photodiodes in a triangular pattern. The pattern of the photodiodes replaces the encoder mask. To reduce the influence of contaminated encoder discs, of unavoidable deviations from an ideal, flat encoder disc surface and variations of the illumination LED due to temperature and age, the two output phase signals are sampled simultaneously at regular intervals. The sampled data are squared and added; the sum should give a constant value, as k(sin2 xcfx86+cos2 xcfx86) should be k regardless of xcfx86. Deviations from the expected value k is interpreted as a fault in the light source and is fed back by adjusting the LED current.
Position detectors that gives a high resolution position information at discrete time intervals, for example once every 100 xcexcs, are quite sufficient for a servo motor controller system that can use this information for controlling the motor torque to obtain the position/velocity profile required. Motor controllers are often used in more complex systems where other devices are dependent on obtaining precise position information using signal protocols and/or requiring data at times not synchronised with the update time of the position detector. There is therefore often a need for transforming periodically available high resolution data to a data stream compatible with the two phase signals obtained from a classical digital output position encoder. The European patent application EP-A1 0 414 953 discloses such a system which however has a very poor resolution of the two phase output signals.
It is an object of the invention to provide a position/velocity transducer that has a very high resolution.
It is another object of the invention to provide a position/velocity transducer that has very small dimensions.
Another object of the invention is to provide a position/velocity transducer that can be integrated into a motor with very limited increase of the dimensions of the combined motor-encoder assembly compared to the size of the motor.
It is another object of the invention to provide a position/velocity transducer having a high resonance frequency in mechanical coupling of the motor rotor to the transducer system.
It is another object of the invention to provide a position/velocity transducer that permits a high precision.
It is another object of the invention to provide a position/velocity transducer that permits a high resolution even when mounted directly on the motor shaft and thus using the motor bearings instead of having own bearings.
It is another purpose of the invention to provide an encoder signal processing method that has a very high resolution and still permits the use of primary signals that have shapes that deviate from a sinusoidal shape with low distortion, that is have a very low intensity of upper harmonics.
It is another object of the invention to provide an encoder signal processing method that permits the use of primary signals that have amplitudes that vary depending on the angle of the encoder.
It is another object of the invention to provide an encoder signal processing method that permits a high resolution of position at specified points of time, for example once or twice for every position control loop in a servo system.
It is another object of the invention to provide an encoder signal processing method that permits a high precision even when using optical elements of limited quality.
It is another object of the invention to provide an encoder signal processing method that permits a high precision obtained from signals that vary in shape from one encoder to another in the same batch.
It is another object of the invention to provide a position/velocity transducer system that gives output signals compatible with the signals from a high quality digital output optical encoder.
It is another object of the invention to use time discrete position information to provide an output data stream that is compatible with data from an incremental encoder having a customer specified number of periods for a full turn.
It is another object of the invention is to use time discrete position information to provide an output data stream that is compatible with data from an incremental encoder having not only a customer specified integral number of periods for a full turn, but also permitting virtual incremental encoders having a non-integer number of periods per turn.
It is another object of the invention to use time discrete position information to provide an output data stream that is compatible with data from an incremental encoder having a customer specified zero pulse.
The objects above are achieved by the invention, the characteristics and features of which appear from the appended claims.
The invention provides a position and velocity transducer that has a high speed resolution and a fast speed response and that is very compact and operates with modest demands on the precision of the bearings. The transducer is based on an optical encoder. When assembled in motors, the encoder disc is assembled on a short protruding part of the rotor close to the torque creating parts. The transducer thus has a very rigid and space saving connection between encoder and motor rotor that permits a high resonance frequency of the mechanical system rotor/encoder. This arrangement is made possible by making one part of the optical system, normally the light emitter part, insertable between rotor and the encoder disc after the introduction of the rotor-encoder disc assembly into the motor stator case. The optical incremental encoder signals are fed both to a moderate frequency digital counter that counts 4 counts per basic encoder period and to an A/D-converter that samples and digitises the primary encoder analog signals. A synchronising network ensures that the primary encoded digitised signals correspond to the correct state of the digital counter. The sampled and digitised analog signals are read by a processor using a linearisation algorithm which yields position information that locally is highly continuous thus permitting good speed estimation also for low speeds when a difference is calculated using position data representing rotor positions very close to each other. If so required, the transducer can be calibrated. The data from such a calibration permits a more precise algorithm that yields position information that is highly linear over the whole active range of the transducer. In order to give outputs compatible with presently commonly used multi-axis controllers, a digital logic network is used. This network is supplied with periodically calculated position and speed data for transforming these data to a high resolution signal that is compatible with that of a high quality optical incremental encoder. Optionally, more than one decoder head can be used to compensate for the bearing play common in servo motors having varying axial loads. In the case position information during power off is required, the light emitter part can be divided into filament or tungsten lamps or LED""s that are operating continuously during power on and that illuminates the high resolution encoder, and LED devices that illuminate a very low resolution track and that in low power mode are pulsed on only at short intervals to permit tracking of slow motor movements while the main power is off. The low resolution track can advantageously also be used to obtain commutation information at power up of the motor.
A position encoder thus has one or several light sources and a movable encoder disc with at least one track having a multitude of adjacent fields of high and low transparency. An encoder mask is arranged having at least two openings, each opening having several adjacent fields of high and low transparency normally with the same pitch as the fields of the movable encoder disc but may have a pitch deviating from the pitch of the movable encoder disc in order to obtain better signal shape, the movable encoder disc and the encoder mask being separated by a very short distance. Further, light detectors are placed so that light from the light source basically must pass through both the encoder disc and an opening in the encoder mask to reach the light detector associated with that opening. The fields of the at least two mask openings are spaced apart relative each other in the normal way for incremental optical encoders so that the light transmitted through the two openings will be basically 90xc2x0 out of phase one to the other. The encoder mask and light detectors are assembled in or on the mechanical part(s) that connects one of the main motor bearings relative to the stator of the motor. The encoder light sources are assembled on a unit that in normal operation mode is mechanically locked to mechanical parts that during the normal operation of the motor are static relative to the mechanical part or parts that connect one of the main motor bearings relative to the stator of the motor. During the assembly of the motor the encoder light source unit is in another position so that the motor can be assembled without the light source touching the encoder disc. After the assembly of the motor, the light source can be moved to and secured in its normal operating position.
More specifically a position transducer is provided for determining the position of a part movable in relation to another part, typically a rotor of an electric machine movable in relation to a stator construction. The transducer comprises an encoder element attached to a first one of the parts, a light source attached to the other, the second one of the parts, and emitting light in paths, where the light is intersected by fields on the encoder element and a light detector attached to the second part and arranged to sense light along the path, which has been intersected by the encoder element. The light source and the light detector is arranged to be moved, in particular rotated, from one position outside an edge of the encoder to a position adjacent to a side of the encoder element, this making a compact layout of the rotor-stator possible.
The light source or the light detector will always comprise some spot or place for emitting or receiving light in the path. This spot or place can then be arranged on an L-shaped support structure having an arm which projects from a shaft. The shaft can be rotated in an opening in the second part and secured in a suitable position.
An encoder mask may be attached to the second part to cooperate with the encoder disc and it then, in the common way, has openings, through which light along the paths passes. To reduce stray light an opaque element, generally having flat shape, is placed between the light detector and the encoder mask in the light paths. It has an opening for each opening in the mask, so arranged that each such opening in the opaque element permits light to pass therethrough to a light detector associated with the corresponding opening in the mask. The openings in the opaque element should then have such a length in the direction of the light paths, that they block light from any opening except the opening associated with a certain light detector to reach that light detector. In particular the openings can be made in a plate or disc-shaped opaque material element, the thickness of which is sufficiently large or at least twice that of the width of the mask openings in a direction perpendicular to the longitudinal or radial direction of the fields of different transparency, these fields being very narrow for a high resolution.
At least walls of the openings in the opaque element may then be blackened to increase the absorption of light that hits the hole walls.
Electronic circuits of the transducer can be assembled with a low thermal impedance path to a heat conductive part that faces the ambient air, generally some front or rear part of the motor. The electronic circuits can also be assembled with a high thermal impedance path between the electronic circuits and a heat conductive part that have a low thermal impedance to heat generating parts of the motor. The electronic circuits of the transducer can further be assembled with a low thermal impedance path to a heat conductive part that is cooled by cooling fins located thereon. The electronic circuits can be assembled with a low thermal impedance path to a heat conductive part that is cooled by air from a fan mounted on the shaft of the rotor. The electronic circuits transducer can finally be assembled with a low thermal impedance path to a heat conductive part that is cooled by liquid cooling, channels therefor being arranged in the heat conductive part, e.g. in the front or rear part or shield of the motor.
The light source can comprise light elements for various uses, such as for continuous illumination and they can then be lamps of the filament type or resistance heated type. Also light elements for intermittent use can be arranged requiring a significantly smaller electric power for their operation than the continuous illumination lamps. A control device will then active the intermittent light elements only when the mains supply is disrupted or for producing light pulses for special purposes.
The transducer can comprise at least two detector devices, each one reading one encoder disc track with a low resolution. These two devices may then be located and arranged so that they have a substantial phase difference. These two devices are arranged to be, during a low power mode, energised with a low duty cycle, and therefore these two devices are connected to a signal processing device, in particular a counter or a microprocessor, which arranged to interpret the possible state changes of the two devices in order to sense movements of the encoder disc.
The at least two detector devices can be arranged to read two encoder disc tracks with the same low resolution. The low resolution track or tracks can also be used for commutation control of the electric motor.
There may be in the transducer at least two zero pulse patterns. The low resolution track detector device signals concurrent with the occurrence of a valid zero pulse are different for the different zero pulse patterns, thereby enabling signal interpreting devices to identify which of the zero pulse patterns that caused the zero pulse signal.
The particular arrangement can also be described by saying that a partially transparent encoder disc is assembled on the rotor shaft between the torque producing air gap of the motor and one of the rotor bearings and that a light source is placed on one side of the encoder disc and light detector devices are placed on the opposite side of the encoder disc, this producing a most compact construction.
The rotor may then comprise a main rotor part, so that the air gap is located at some surface thereof, e.g. an exterior cylindrical surface. Then there is a side surface of the main rotor part which is connected to the first surface and here the encoder disc is mounted, inside a bearing for the rotor. A step can be made in the side surface for attaching the encoder disc at the edge portion of the step, when the encoder disc has an annular shape. Such a mounting will achieve that a marginal circumferential portion of the encoder is freely available, located at distance from the side surface and here the encoder tracks are located to be read by a detector.
The position transducer can also be mounted in a motor in another way, where the motor comprises in the conventional way a rotor having a rotor main part and axles projection therefrom and two rotor bearings located at each side of the rotor main part for rotatably mounting the axles. The encoder disc can be made with a fairly small outer diameter and is rigidly attached to a first one of the rotor axles, for instance at an end surface thereof. The outer diameter can then be smaller than an inner diameter of the bearing which supports this axle allowing that the rotor axle with the encoder disc attached thereto can be inserted through this bearing or that this bearing with an attached side plate or shield plate is moved over and around the axle and the encoder disc when assembling the motor and the position transducer.
In the same manner as above, the first axle can comprise a step for mounting the encoder disc. The encoder disc is then attached to a surface at this step for obtaining that a marginal circumferential portion of the encoder disc is located at distance from surfaces of the rotor and in particular of the first axle.
In another aspect, a partially transparent encoder disc is assembled on a rotor shaft between a torque producing air gap of the motor and one of the rotor bearings, light sources are placed on one side of the encoder disc and light detector devices are placed on the opposite side of the encoder disc as above. Further, there are at least two light detectors groups reading a high resolution track, where the individual detector elements in each of these groups are designed to read signals from the high resolution track with a phase difference of at least 60xc2x0. These at least two detector groups are preferably basically symmetrically oriented around the motor axis and the axis of the encoder disc, and position misreadings caused by radial movements of the encoder disc can then be cancelled or eliminated by adding or averaging the position values calculated from the signals from the different light detectors or simply by averaging or adding the signals.
There may in particular be only two light detector groups and then these two groups should be placed substantially on opposite sides of the motor axis along a diameter thereof, thus approximately 180xc2x0 apart.
There may also be three light detector groups and these may then be substantially symmetrically oriented around the motor axis, thus approximately 120xc2x0 apart.
The electronic processing of the signals from the detector heads may be designed to give an indication having a high accuracy. It presupposes that two analog signals are provided from the detectors, these signals having a considerable phase difference, preferably 90xc2x0. The processor circuits comprise an substantially asynchronous low resolution digital conversion circuit receiving the analog signals to give at least two and preferably four low resolution states for each period of the analog signals, a device arranged to decode the transitions from one of low resolution states to an adjacent state into count up or count down commands, a counter connected to the decoding device, the state of the counter being incremented for each count up command and the state of the counter being decremented for each count down command, an substantially synchronous high resolution analog to digital conversion system that samples the analog signals at regular intervals, and a synchronisation device that samples the state of the counter at a time so selected that the state of the counter represents the values of the analog signals at a time close to the times at which the analog signal were sampled by the high resolution analog to digital conversion systems. Finally there are data processing means to convert the high resolution analog data to position information.
In a position encoder there can then generally be provided a transducer such as the system light source-encoder track-light detector and it produces at least one analog signal which varies periodically with the position of an encoding element, which as above for instance is a an encoder disc having periodically repeated fields having some physically detectable property different from the regions between the fields. A counting circuit receives the analog signal and determines therefrom a value which is stored and represents the position of the encoding element as the number of periods of the encoding element from an initial or a reference position. An analog to digital converter receives also the analog signal for producing a digital value being a sample of the analog signal. Evaluation means determine the position of the encoder element by combining the digital data from the A/D-converter with the value stored the counting circuit. The digital data will provide a position value substantially within one encoder period, thus accomplishing together the value stored in the counting circuit an accurate indication of the position.
In particular the transducer can comprise an incremental encoder producing two analog signals that each one varies in a periodic manner with the position of the encoding element. Further the analog signals should have a considerable phase difference, such as substantially 90xc2x0 or at least in the range 60-120xc2x0.
The data processing means may also be described as comprising first processing means for conversion of the high resolution analog to digital conversion data to information on the position within a basic encoder period and second processing means for combining the information on the position with an encoder period given by the processed high resolution data with the information on the position as expressed as a multitude of encoder periods given by the sampled counter data.
Further the fine indications of the position provided by the position transducer can calibrated and then the transducer is assumed generally to comprise a modulating device such as the track on an encoder disc, some means for producing at least one analog signal varying periodically with the position of the modulating device and an analog to digital conversion system for converting the analog signal into digital information at repeated times, e.g. at regularly repeated times. The digital information will then generally represent the position of the modulating device within at least one half period of the modulating device, the periods thus being for instance the periods of the fields of the encoder track, that is the distance between centres of adjacent fields. Digital computing means can then process the digital information for transforming it into digital values representing digitally a sinus wave or curve. Therefor the digital computing means comprise means for storing information available on deviations of the periodic variation of the analog signal from a sinusoidal shape which has a constant amplitude for all the periods of the modulating device.
The information stored in the storage means of the digital computing means may include information on variations of amplitude of the periodically varying analog signal for different whole periods of the movement of the modulating device. The information stored in the storage means of the digital computing means may also include information on the systematic deviation of the periodically varying analog signal from a sinusoidal shape.
The modulating device can be an optical encoder disc or bar having a periodically varying reflection. It can also be a disc or bar having periodically varying magnetisation and the magnetic flux in the vicinity of the disc or can be recorded by a hall sensors for producing the analog signal.
The transducer can also use magnetic flux recorded by a hall sensor situated in a magnetic circuit, the flux of which is driven by a permanent magnet and the flux intensity of which depends on a movable ferromagnetic object. This ferromagnetic object can then be designed so that the magnetic permeability of the magnetic circuit including the permanent magnet and the hall sensor varies periodically with the position of the movable ferromagnetic object.
An incremental position encoder system can also be designed for providing special output signals suitable for motor control and the system then generally comprises some primary position indication means which provide signals representing a position of some object such as an encoder disc. First data processing means, e.g. a microprocessor, working at a medium or moderate speed calculate high resolution position information from the signals of the indication means at regularly repeated calculation times and they also estimate the change in position from the most recent calculation time to the next calculation time. There are also second data processing means working at a high frequency such as an adder connected to suitable registers for adding repeatedly at a high frequency a value derived from the estimated change in position to a value stored in a register, such as a hardware register, or an accumulator, where this value is selected so that it represents an estimated position.
The second data processing means can instead be arranged to update the register, which can also be a counter or memory location, at regularly repeated times, where these updating times are given by a high frequency divide-by-n counter that divides a high frequency clock signal by a value which is derived from the estimated change in position.
The value of the estimated position stored in the register etc. can be stored in second register at suitable times, so that this value stored in the second register can be accessed by or transferred to various information requesting devices through a data bus, a local network, a serial link controller or other data transfer means.
There is advantageously also a two phase encoder simulation device which is connected to the register or accumulator etc. holding the estimated position. The register etc. may then be arranged to output a signal to the simulation device when the value stored or hold therein passes a predetermined modulo limit such as the maximum integer number which can be represented in the register, that is the signal is then a carry signal, so that the simulation device changes, on receiving the signal, states on its outputs or of its output signals. The limit should be chosen to either fit a resolution required by information requesting devices or to respect limitations given by information requesting devices, in particular a maximum counter counting range or a maximum permissible encoder signal frequency. Also the high frequency of the adding operation can be adjusted for this fit or for respecting the limitations.
The case using a signal representing some carry can be described in the following way. The least significant bits of the register or memory location storing or holding the estimated position value is a register or memory location that will obtain overflow or underflow several times for each full range movement of the position transducer. Each such overflow or underflow event will then cause a change of the state of the outputs of the two phase encoder signal simulation device.
In the case where the high speed second data processing means updates an estimated position at intervals given by a high frequency divide-by-n counter that divides a high frequency clock with a value derived from the estimated change in position, the updating of the estimated position can be performed caused by a change of the state of the outputs of the two phase encoder signal simulation device.
The first data processing means may advantageously obtain information permitting a comparison of the position according to the primary position indication means and the data representing the estimated position and then information can be provided which is suitable for correction of differences between the position according to the primary position indication means and the data representing the estimated position.